I don't use Mastercam, but it is a fairly simple manual task if you want to try it out. Simply insert the proper Z values at the places where you want to start and end the slope.
Code:
G21 (mm)
M6 T1 (Change Tool: Diameter: 3.0000 mm)
M3 (Start Spindle)
G0 Z2.0000
G0 X0.0000 Y0.0000
G0 X1.5000 Y1.5000 (Origin 0)
G1 Z0.0000 F50.0 (Starting depth)
G1 X98.5000 Y1.5000 Z-2.0000 F100.0 (leg 1)
G1 X98.5000 Y3.0000 F100.0 (leg 2)
G1 X1.5000 Y3.0000 Z0 F100.0 (leg 3)
G1 X1.5000 Y4.5000 F100.0 (leg 4)
G1 X98.5000 Y4.5000 Z-2 F100.0 (leg 1)
G1 X98.5000 Y6.0000 F100.0 (leg 2)
G1 X1.5000 Y6.0000 Z0 F100.0 (leg 3)
G1 X1.5000 Y7.5000 F100.0 (leg 4)
G1 X98.5000 Y7.5000 Z-2 F100.0 (leg 1)
G1 X98.5000 Y8.5000 (x-line last pass)
G1 X1.5000 Y8.5000 Z0 (y-line last pass)
(Next pass will be the same pattern down to Z -4)
G0 X1.5000 Y1.5000 (Origin 0)
G1 Z0.0000 F50.0 (Starting depth)
G1 X98.5000 Y1.5000 Z-4 F100.0 (leg 1)
G1 X98.5000 Y3.0000 F100.0 (leg 2)
G1 X1.5000 Y3.0000 Z0 F100.0 (leg 3)
G1 X1.5000 Y4.5000 F100.0 (leg 4)
G1 X98.5000 Y4.5000 Z-4 F100.0 (leg 1)
G1 X98.5000 Y6.0000 F100.0 (leg 2)
G1 X1.5000 Y6.0000 Z0 F100.0 (leg 3)
G1 X1.5000 Y7.5000 F100.0 (leg 4)
G1 X98.5000 Y7.5000 Z-4 F100.0 (leg 1)
G1 X98.5000 Y8.5000 (x-line last pass)
G1 X1.5000 Y8.5000 Z0 (y-line last pass)
(Last pass will be the same pattern down to Z -6)
G0 X1.5000 Y1.5000 (Origin 0)
G1 Z0.0000 F50.0 (Starting depth)
G1 X98.5000 Y1.5000 Z-6 F100.0 (leg 1)
G1 X98.5000 Y3.0000 F100.0 (leg 2)
G1 X1.5000 Y3.0000 Z0 F100.0 (leg 3)
G1 X1.5000 Y4.5000 F100.0 (leg 4)
G1 X98.5000 Y4.5000 Z-6 F100.0 (leg 1)
G1 X98.5000 Y6.0000 F100.0 (leg 2)
G1 X1.5000 Y6.0000 Z0 F100.0 (leg 3)
G1 X1.5000 Y7.5000 F100.0 (leg 4)
G1 X98.5000 Y7.5000 Z-6 F100.0 (leg 1)
G1 X98.5000 Y8.5000 (x-line last pass)
G1 X1.5000 Y8.5000 Z0 (y-line last pass)
(Now you have a sloping pocket in a 100 x 10 box, slope is from 0 to -6)
G0 Z2.0000 (Retract to safe height)
M5 (Stop Spindle)
G0 X0 Y0 (Origin)
M30 (End of Program)
As an example, the above G code will result in a pocket with 100 x 10 size and the slope will start from Z 0 and end at Z -6. It runs in three passes, basically pass 2 and 3 are copies of pass 1, only the Z depth value is changed. I also include an XZ profile picture of the passes to show how it would look like if run on my CNC.
Attachment 309602
I don't know if this was of any help, but have a look at it and ask again if it did not solve your problems. You can also post your pocket G code and we could try to modify it together.